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Is obesity contagious?

By Roxanne Khamsi

They say it’s not what you know but who you know. A new study suggests that this aphorism might hold true when it comes to your body weight.

If a friend of yours becomes obese, you have a nearly 60% higher chance of sliding into this category as well, according to the analysis. The finding has prompted researchers to call obesity a “socially contagious” disease in which a sense of what constitutes a normal body weight passes from one person to the next.

Around the world, the number of people struggling with weight problems is increasing rapidly. Within the US, for example, the fraction of the population considered obese has doubled in the past 25 years, from 15% to 32%. Based on World Health Organization definitions, people are considered obese if they have a body mass index, or BMI, over 30. BMI is derived by dividing an individual’s weight (in kilograms) by the square of his height (in metres) – calculate your BMI here.

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But a new analysis from James Fowler at the University of California in San Diego and Nicholas Christakis at the Harvard Medical School in Boston, Massachusetts, both in the US, suggests that the spread of social norms rather than just behavioural changes might explain the rise in obesity.

The researchers analysed archived data on body weight dating back to 1971 that was collected as part of the Framingham Heart Study, a large-scale, long-term study designed to examine the factors that influence cardiovascular health.

What’s more, they mapped the network of relationships between people involved in the trial. This was made possible by the fact that when participants in the trial came in for a check-up every few years, they also provided an updated list of their family and friends who could help doctors locate them in the future, if need be.

Dense network

Researchers searched through the database to find whether any of these family members or friends also participated in the trial. In the end, Fowler and Christakis succeeded in mapping a densely interconnected social network involving more than 12,000 study subjects.

After controlling for other factors, such as gender, normal weight gain with age and socioeconomic status, they found that among adults, those with siblings who become obese have a 40% higher chance of also becoming obese themselves. Moreover, a person whose spouse becomes obese has a 37% higher chance of moving into this weight category as well.

But friends seemed to exert the greatest influence on body weight&colon; Even after controlling for the fact that people tend to form bonds with others similar to them, scientists found that an individual is 57% more likely to become obese within an interval of the study – about two to four years – if someone he or she listed as a friend becomes obese during the same time period.

On average, an individual gained 2.3 kilograms (5 pounds) during this time period if a person they listed as a friend became obese. Scientists stress that this weight gain occurred after just a few years and would accumulate rapidly over time, but add that the amount does not account for confounding factors such as gender and height.

The effect among mutual friends was even bigger. If both people listed each other as friends, and one became obese first, the second was about three times as likely to follow suit.

Long distances

Fowler notes that friends living nearby seemed to exert just as much influence on a person’s weight as those living 800 kilometres (500 miles) away. To him, this suggests that the trend has more to do with a spread of social norms than behaviours. In other words, the idea of what constitutes a normal weight travels more easily across distance than do behaviours – such as exercise and eating routines.

Researchers also believe that the opposite holds true&colon; people whose friends lose weight over time are also more likely to become thinner. But they stress that this should not by any means make people ditch their heavier friends.

Instead, they say that the findings should encourage people who want to lose weight to motivate their friends to do the same. “You want to act in concert with your friends,” says Christakis.

Small town effect

There could be power in knowing that friends can exert an influence on one’s weight. “It may be that simply realising this is enough to mute the influence,” says Richard Suzman of the National Institute on Aging in Bethesda, Maryland, US. He says the study will change how people think about supposedly non-contagious diseases.

Interestingly, the researchers found that obesity was socially contagious across three degrees of separation (so, for example, a friend of a friend who is obese also has an increased risk of obesity).

The authors say this might mean that the problem of obesity could spread more quickly through small towns – where social networks tend to overlap more than in big cities.

Researchers say that they plan to examine the data to better understand whether other traits influencing health, such as drinking and smoking, spread across multiple degrees of separation.